System software interfaces for space-optimized block devices

ABSTRACT

Interfaces to storage devices that employ storage space optimization technologies, such as thin provisioning, are configured to enable the benefits gained from such technologies to be sustained. Such an interface may be provided in a hypervisor of a virtualized computer system to enable the hypervisor to discover features of a logical unit number (LUN), such as whether or not the LUN is thinly provisioned, and also in a virtual machine (VM) of the virtualized computer system to enable the VM to discover features of a virtual disk, such as whether or not the virtual disk is thinly provisioned. The discovery of these features enables the hypervisor or the VM to instruct the underlying storage device to carry out certain operations such as an operation to deallocate blocks previously allocated to a logical block device, so that the storage device can continue to benefit from storage space optimization technologies implemented therein.

CROSS-REFERENCE TO RELATED APPLICATION(S)

This application claims the benefit of U.S. Provisional PatentApplication No. 61/378,076, filed Aug. 30, 2010, the entire contents ofwhich are incorporated by reference herein.

BACKGROUND

Computer virtualization is a technique that involves encapsulating aphysical computing machine platform into a virtual machine that isexecuted under the control of virtualization software running on ahardware computing platform, or “host.” A virtual machine has bothvirtual system hardware and guest operating system software. Virtualsystem hardware typically includes at least one “virtual disk,” a singlefile or a set of files that appear as a typical storage drive to theguest operating system. The virtual disk may be stored on the hostplatform or on a remote storage device. Typically, a virtual machine(VM) uses the virtual disk in the same manner that a physical storagedrive is used, to store the guest operating system, applicationprograms, and application data.

The virtualization software, also referred to as a hypervisor, managesthe guest operating system's access to the virtual disk and maps thevirtual disk to the underlying physical storage resources that reside onthe host platform or in a remote storage device, such as a storage areanetwork (SAN) or network attached storage (NAS). Because multiplevirtual machines can be instantiated on a single host, allocatingphysical storage space for virtual disks corresponding to everyinstantiated virtual machine in an organization's data center can stressthe physical storage space capacity of the data center. For example,when provisioning a virtual disk for a virtual machine, thevirtualization software may allocate all the physical disk space for thevirtual disk at the time the virtual disk is initially created,sometimes creating a number of empty data blocks containing only zeros(“zero blocks”). However, such an allocation may result in storageinefficiencies because the physical storage space allocated for thevirtual disk may not be timely used (or ever used) by the virtualmachine. In one solution, known as “thin provisioning,” thevirtualization software dynamically allocates physical storage space toa virtual disk only when such physical storage space is actually neededby the virtual machine and not necessarily when the virtual disk isinitially created.

In a similar manner, thin provisioning may be implemented as a storagespace optimization technology in the underlying storage hardware, e.g.,storage array, which may include an array of rotating disks or solidstate disks as the physical storage media. In such cases, a storagesystem controller that manages the physical storage media and exposesthem as logical data storage units, referred to as logical unit numbers(LUNs), to the host, thinly provisions the LUNs. That is, the storagesystem controller dynamically allocates physical storage space to theLUNs only when such physical storage space is actually needed by theLUNs and not necessarily when the LUNs are initially created. As aresult, when the LUNs are initially created, the logical size of each ofthe LUNs is typically much greater than its physical size.

However, even with the use of thinly-provisioned virtual disks andthinly-provisioned LUNs, storage inefficiencies may be caused by anaccumulation of “stale” data, i.e., disk blocks that were previouslyused and are currently unused but remain allocated. For example,deletion of a file, such as a temporary file created as a backup duringediting of a document, in the virtual disk by the guest operating systemdoes not generally result in a release of the actual data blockscorresponding to the temporary file. While the guest operating systemmay itself track the freed data blocks relating to the deleted temporaryfile in its own guest file system (e.g., by clearing bits in a bitmapfor the guest file system), the guest operating system is not aware thatthe disk on which it has deleted the temporary data file is actually a“virtual disk” that is itself a file. Therefore, although a portion(i.e., the portion of the virtual disk that stores the guest filesystem's bitmap of freed data blocks) of the virtual disk may bemodified upon a deletion of the temporary file by the guest operatingsystem, the portion of the virtual disk corresponding to actual datablocks of the deleted temporary file does not actually get released fromthe virtual disk back to the LUN by the virtualization software. Thisbehavior can result in storage inefficiencies because such “stale”portions of the virtual disk are not utilized by the corresponding guestoperating system and are also not available to the virtualizationsoftware for alternative uses (e.g., reallocated as part of a differentvirtual disk for a different virtual machine, etc.).

The process known as Storage vMotion™ involving live migration ofvirtual machine disk files (including one or more virtual disks andother VM configuration files) from a source LUN to a destination LUNprovides another example of “stale” data being accumulated in athinly-provisioned LUN. During Storage vMotion™, actual data blockscorresponding to the virtual machine disk files are copied from thesource LUN to the destination LUN, and at the conclusion of the copying,the LUN supporting the VM is atomically switched from the source LUN tothe destination LUN. After the atomic switch-over, the actual datablocks corresponding to the virtual machine disk files in the source LUNare no longer needed. While the virtualization software may itself trackthese data blocks and mark them as “free,” for example, by actuallydeleting the virtual machine disk file from the source LUN, the portionof the source LUN corresponding to these free data blocks of the virtualmachine disk file does not actually get released from the LUN back tothe storage array. This may be acceptable if the virtualization softwarequickly reallocates the freed data blocks in the source LUN foralternative uses (e.g., by allocating a new virtual machine disk filefor another virtual machine, etc.). However, in cases where the freeddata blocks remain unallocated, such “stale” portions of the LUN lessenthe storage space efficiencies gained from thin provisioning (e.g.,since such stale portions could have been reallocated by the storagearray manager to a different thinly provisioned LUN that may beexperiencing storage pressure).

SUMMARY

One or more embodiments of the present invention provide system softwareinterfaces to storage devices that employ storage space optimizationtechnologies, such as thin provisioning, to enable the benefits gainedfrom such technologies to be sustained. Such an interface may beprovided in a hypervisor of a virtualized computer system to enable thehypervisor to discover features of a LUN, such as whether or not the LUNis thinly provisioned, and also in a VM of the virtualized computersystem to enable the VM to discover features of a virtual disk, such aswhether or not the virtual disk is thinly provisioned. The discovery ofthese features enables the hypervisor or the VM to instruct theunderlying storage device to carry out certain operations such as anoperation to deallocate blocks previously allocated to a logical blockdevice, i.e., the LUN or the virtual disk, so that the underlyingstorage device can continue to benefit from storage space optimizationtechnologies implemented therein.

A method of determining features of a logical block device for acomputer system, according to an embodiment of the present invention,includes the steps of: querying the logical block device with a firstquery and receiving a response thereto from the logical block devicethat indicates whether or not the logical block device is thinlyprovisioned; and querying the logical block device with a second queryand receiving a response thereto from the logical block device thatindicates whether or not the logical block device supports an unmapcommand, wherein the unmap command is a command to deallocate blockspreviously allocated to the logical block device.

Embodiments of the present invention further include a non-transitorycomputer-readable storage medium storing instructions that when executedby a computer system cause the computer system to perform the method setforth above.

A computer system, according to an embodiment of the present invention,includes a system software component that interfaces the computer systemto a logical block device, a processor, and memory. The processor isprogrammed to query the logical block device with a first query andreceive a response thereto from the logical block device that indicateswhether or not the logical block device is thinly provisioned, and querythe logical block device with a second query and receive a responsethereto from the logical block device that indicates whether or not thelogical block device supports an unmap command, wherein the unmapcommand is a command to deallocate blocks previously allocated to thelogical block device.

A computer system, according to another embodiment of the presentinvention, includes a plurality of hosts and a management server forvirtual machines running in the hosts. The hosts comprise of first andsecond hosts each having virtualization software of a first type thatsupports thinly provisioned virtual disks and an unmap command, whereinthe unmap command is a command for deallocating blocks previouslyallocated to a thinly provisioned virtual disk, and a third host havingvirtualization software of a second type that does not support thinlyprovisioned virtual disks or the unmap command. The management serverincludes a resource scheduling software executed therein to recommendlive migration of virtual machines between two hosts other than from ahost having a virtualization software of the first type to a host havinga virtualization software of the second type.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram that shows a virtualized computer architectureaccording to one or more embodiments.

FIG. 2A depicts a block diagram representative of a server in thevirtualized computer architecture of FIG. 1, according to one or moreembodiments.

FIG. 2B depicts block diagram representative of a storage array in thevirtualized computer architecture of FIG. 1, according to one or moreembodiments.

FIG. 3 is a flow diagram illustrating a method to collect configurationcharacteristics of a LUN that operates as a logical block device of ahypervisor running in the server of FIG. 2A.

FIG. 4 is a flow diagram illustrating a method to collect configurationcharacteristics of a virtual disk that operates as a logical blockdevice of a guest operating system of a virtual machine running in theserver of FIG. 2A.

FIG. 5 is a flow diagram that illustrates a method of storage spacereclamation from a virtual disk.

FIG. 6 is a flow diagram that illustrates a method of storage spacereclamation from a LUN that is initiated by a hypervisor in response toa command from a VM.

FIG. 7 is a flow diagram that illustrates a method of storage spacereclamation from a LUN that is initiated by a hypervisor in response toa command from a management server.

FIG. 8 is a flow diagram that illustrates a method of performingcompatibility checks prior to initiating a process to live migrate a VM.

FIG. 9 is a flow diagram that illustrates a method of notifying amanagement server of a virtualized computer system that a LUN hasreached a certain threshold in used capacity.

FIG. 10 is a flow diagram that illustrates a method of performingcorrective measures by a management server upon receiving thenotification in FIG. 9.

FIG. 11 is a flow diagram that illustrates a method of detecting andhandling an error caused when a LUN runs out of space while performing awrite operation.

FIG. 12 is a flow diagram that illustrates a method of retroactivelyreclaiming storage space from a LUN.

DETAILED DESCRIPTION

FIG. 1 is a block diagram that shows a virtualized computer architecture100 according to one or more embodiments. Virtualized computerarchitecture 100 includes a plurality of servers 110 connected throughnetwork 120 to a shared storage system that includes one or more storagearrays 130. There may be any number of servers 110, each of which maycomprise a general purpose computer system having one or more virtualmachines accessing data stored on any number of storage arrays 130.Network 120 may be a wide area network, a local area network, or anetwork hosting a protocol especially suited for storage arrays 130,such as Fibre Channel, iSCSI, etc., and may comprise one or more ofswitches. Storage arrays 130 may be of any type such as anetwork-attached storage (NAS) filer or a block-based device over astorage area network (SAN). While storage arrays 130 are typically madeup of a plurality of disks, it should be recognized that as prices forsolid-state non-volatile storage devices fall, they are increasinglytaking the place of rotating disk storage media. The use of the term,“disk” herein, should therefore not be construed as limited only torotating disk storage media, but also what is become known as solidstate disks, or “SSDs.”

Virtualized computer architecture 100 is managed by a management server148, which is a computer program that resides and executes in a centralserver or alternatively, in one of servers 110. Management server 148 isin communication with each of servers 110, and carries outadministrative tasks for virtualized computer architecture 100 such asload balancing between servers 110 and workload balancing betweenstorage arrays 130.

FIGS. 2A and 2B respectively depict block diagrams of a server 200 thatis representive of any of servers 110 and a storage array 250 that isrepresentative of any of storage arrays 130, according to one or moreembodiments. Server 200 may be constructed on a conventional, typicallyserver-class, hardware platform 202. As shown in FIG. 2A, server 200includes HBAs 204 and NIC 201 that enable server 200 to connect tostorage array 250. As further shown in FIG. 2A, hypervisor 208 isinstalled on top of hardware platform 202 and it supports a virtualmachine execution space 210 within which multiple virtual machines (VMs)212 ₁-212 _(N) may be concurrently instantiated and executed. Each suchvirtual machine 212 ₁-212 _(N) implements a virtual hardware platform214 that supports the installation of a guest operating system (OS) 216which is capable of executing applications 218. Examples of a guest OS216 include any of the well-known commodity operating systems, such asMicrosoft Windows, Linux, and the like. In each instance, guest OS 216includes a native file system layer (not shown in FIG. 2A), for example,either an NTFS or an ext3FS type file system layer. These file systemlayers interface with virtual hardware platforms 214 to access, from theperspective of guest operating systems 216, a data storage HBA, which inreality, is virtual HBA 220 implemented by virtual hardware platform 214that provides the appearance of disk storage support (in reality,virtual disks or virtual disks 222 _(A)-222 _(X)) to enable execution ofguest OS 216 transparent to the virtualization of the system hardware.In certain embodiments, virtual disks 222 _(A)-222 _(X) may be thinlyprovisioned and appear to support, from the perspective of guest OS 216,the SCSI standard for connecting to the virtual machine or any otherappropriate hardware connection interface standard known to those withordinary skill in the art, including IDE, ATA, and ATAPI.

Although, from the perspective of guest operating systems 216, filesystem calls initiated by such guest operating systems 216 to implementfile system-related data transfer and control operations appear to berouted to virtual disks 222 _(A)-222 _(X) for final execution, inreality, such calls are processed and passed through virtual HBA 220 toadjunct virtual machine monitor (VMM) layers 224 ₁-224 _(N) thatimplement the virtual system support needed to coordinate operation withhypervisor 208. In particular, HBA emulator 226 functionally enables thedata transfer and control operations to be correctly handled byhypervisor 208 which ultimately passes such operations through itsvarious layers to true HBAs 204 or NIC 201 that connect to storage array250. Assuming a SCSI supported virtual device implementation (althoughthose with ordinary skill in the art will recognize the option of usingother hardware interface standards), SCSI virtualization layer 228 ofhypervisor 208 receives a data transfer and control operation (in theform of SCSI commands, for example, intended for a SCSI-compliantvirtual disk) from VMM layers 224 ₁-224 _(N), and converts them intofile system operations that are understood by virtual machine filesystem (VMFS) 230 in order to access a file stored in one of the LUNs instorage array 250 under the management of VMFS 230 that represents theSCSI-compliant virtual disk. In one embodiment, the file representingthe virtual disk conforms to the VMware Virtual Disk (VMDK) file formatpromulgated by VMware, Inc. for virtual disks, although it should berecognized that alternative virtual disk file formats may be used inother embodiments.

SCSI virtualization layer 228 then issues these file system operationsto VMFS 230. VMFS 230, in general, manages creation, use, and deletionof files (e.g., such as .vmdk files representing virtual disks) storedon LUNs exposed by storage array 250. One example of a clustered filesystem that can serve as VMFS 230 in an embodiment is described in U.S.Pat. No. 7,849,098, entitled “Multiple Concurrent Access to a FileSystem,” filed Feb. 4, 2004 and issued on Dec. 7, 2010, the entirecontents of which are incorporated by reference herein. VMFS 230,converts the file system operations received from SCSI virtualizationlayer 228 to volume (e.g. LUN) block operations, and provides the volumeblock operations to logical volume manager 232. Logical volume manager(LVM) 232 is typically implemented as an intermediate layer between thedriver and file system layers, and supports volume orientedvirtualization and management of the LUNs accessible through HBAs 204and NIC 201. LVM 232 issues raw SCSI operations to device access layer234 based on the LUN block operations. Data access layer 240 includesdevice access layer 234, which discovers storage array 250, and appliescommand queuing and scheduling policies to the raw SCSI operations, anddevice driver 236, which understands the input/output interface of HBAs204 and NIC 201 interfacing with storage array 250, and sends the rawSCSI operations from device access layer 234 to HBAs 204 or NIC 201 tobe forwarded to storage array 250.

It should be recognized that the various terms, layers andcategorizations used to describe the virtualization components in FIG.2A may be referred to differently without departing from theirfunctionality or the spirit or scope of the invention. For example, VMMs224 may be considered separate virtualization components between VMs 212and hypervisor 208 (which, in such a conception, may itself beconsidered a virtualization “kernel” component) since there exists aseparate VMM for each instantiated VM. Alternatively, each VMM may beconsidered to be a component of its corresponding virtual machine sincesuch VMM includes the hardware emulation components for the virtualmachine. In such an alternative conception, for example, the conceptuallayer described as virtual hardware platform 214 may be merged with andinto VMM 224 such that virtual host bus adapter 220 is removed from FIG.2A (i.e., since its functionality is effectuated by host bus adapteremulator 226).

Storage array manager 251 of storage array 250, as depicted in FIG. 2B,receives the raw SCSI operations corresponding to one of its LUNs andresolves them into the appropriate extents within the spindles ofstorage array 250 that are operated upon. Storage array manager 251,which represents one or more programmed storage processors, generallyserves as a communication agent (to the outside world) for storage array250, and implements a virtualization of physical, typically diskdrive-based storage units, referred to in FIG. 2B as spindles 252_(A)-252 _(N), that reside in storage array 250. From a logicalperspective, each of these spindles can be thought of as a sequentialarray of fixed sized extents 254. Storage array manager 251 abstractsaway complexities of targeting read and write operations to addresses ofthe actual spindles and extents of the disk drives by exposing to server200 an ability to view the aggregate physical storage space provided bythe disk drives as a contiguous logical storage space that may bedivided into a set of virtual SCSI block devices previously referred toherein as LUNs 256 _(A)-256 _(M) (“Logical Unit Numbers”). Thevirtualization of spindles 252 _(A)-252 _(N) into such a contiguouslogical storage space of LUNs 256 _(A)-256 _(M) can provide a moreefficient utilization of the aggregate physical storage space that isrepresented by an address space of a logical volume. Storage arraymanager 251 maintains metadata 255 that includes a mapping (hereinafter,also referred to as an extent-mapping) for each of LUNs 256 _(A)-256_(M) to an ordered list of extents, wherein each such extent can beidentified as a spindle-extent pair <spindle #, extent #> and maytherefore be located in any of the various spindles 252 _(A)-252 _(N).

In certain embodiments, storage array 250 may employ a storage spaceoptimization technology called “thin provisioning” when allocating LUNs.When a LUN is “thinly” provisioned, the logical size of the LUN asreported by storage array 250 may be larger than the amount of physicalspace initially backing that LUN. All consumers of the LUN only see thelogical size of the LUN. As write operations are issued to previouslyunallocated blocks of a thin-provisioned LUN, the amount of actualphysical space consumed grows, and at some point, the LUN may run out ofphysical space. In a similar fashion, in a virtualization environmentsuch as that depicted in FIG. 2A, a virtual disk 222 stored on a LUN ofstorage array 250 may be configured to be “thinly provisioned,” forexample, by hypervisor 208 (or by management server 148 in certainembodiments). From the perspective of guest OS 216, such athinly-provisioned virtual disk 222 would be perceived as having a fixedlogical size, but, in reality, VMFS 230 allocates LUN storage space tovirtual disk 222 (e.g., a .vmdk file) dynamically, such that at anygiven time, the actual storage space in the LUN that backs virtual disk222 may be less than the logical size.

FIG. 3 is a flow diagram illustrating a method carried out by hypervisor208 to collect configuration characteristics of a LUN, which operates asa logical block device for hypervisor 208. These configurationcharacteristics of the LUN may be used in techniques described below to“reclaim” storage space from the LUN (back to a storage systemsupporting the LUN, such as storage array 250) by issuing an “UNMAP”command to the LUN. As depicted in FIG. 3, at step 302, hypervisor 208issues a SCSI Read Capacity command (e.g., 16 bit version of thecommand) to the LUN. The response of the LUN, received at step 304,includes an indication of whether or not the LUN supports thinprovisioning as indicated by the setting of a bit (which, in oneembodiment, is known as the “thin provisioning enabled (TPE) bit”). If,at step 306, hypervisor 208 determines that the LUN supports thinprovisioning (e.g., the TPE bit is set), the method continues on to step308. If, at step 306, hypervisor 208 determines that the LUN does notsupport thin provisioning (e.g., the TPE bit is not set), the methodends.

At step 308, hypervisor 208 records the LUN' s support for thinprovisioning and issues a SCSI Inquiry command (e.g., utilizing the 0xB0“Vital Product Data” code as the type of inquiry in one embodiment) tothe LUN. The response of the LUN, received at step 310 and recorded atstep 312, includes an indication as to whether or not LUN supports an“UNMAP” command (in some embodiments, indicated by a UNMAP “bit”) and,if there is support, the response also includes a report of severalparameters to be used with UNMAP commands. In its simplest form, in oneembodiment, an UNMAP command specifies a list of blocks that are to beunmapped by the LUN and released to the underlying storage systemsupporting the LUN. In one such embodiment, the parameters reportedinclude D_(G), a granularity at which the LUN manages data, D_(offset),an alignment parameter expressed at an offset at which the LUN prefersto receive UNMAP commands, and N_(MAX) _(—) _(D), a maximum number of<offset, length> pairs that can be specified with a single UNMAPcommand. For example, if D_(offset) has a value of 4 KB, then, the LUNwill accept SCSI operations, such as UNMAP commands, that start at amultiple of 4 KB address (e.g., addresses at 0 KB, 4 KB, 8 KB, 12 KB,etc.). If D_(G) then has a value of 512 KB, then the LUN will acceptSCSI operations that specify a block size that is a multiple of 512 KB.In such an example, the LUN would accept an UNMAP command to unmap acontiguous block of 1024 KB that begins at address corresponding to anoffset of 12 KB from the beginning of the LUN, but would not accept anUNMAP command to unmap any contiguous block beginning at an addresscorresponding to an offset of 1 KB, 2 KB, 3 KB, etc. from the beginningof the LUN or where the contiguous block size is less than 512 KB. Itshould be recognized that the values D_(G), D_(offset) and N_(MAX) _(—)_(D) are set or defined by the storage vendor of the underlying storagesystem supporting the LUN.

FIG. 4 is a flow diagram illustrating a method carried out by guest OS216 to collect configuration characteristics of a virtual disk 222,which operates as a logical block device of guest OS 216 of virtualmachine 212. As should be recognized, FIG. 4 repeats the same steps asFIG. 3, except that rather than a hypervisor querying a LUN in a storagearray for its configuration characteristics as in FIG. 3, in FIG. 4, aprocess in virtual machine 212 (e.g., a user level process developed totrack free blocks in guest OS 216 or a configuration routine in a SCSIdevice driver loaded into guest OS 216, etc.) is querying virtual disk222 (e.g., an emulated SCSI device whose data is actually stored as afile in the LUN) for its configuration characteristics. Because thevirtual disk is being emulated by hypervisor 208, hypervisor 208provides the virtual disk's configuration characteristics (on behalf ofthe virtual disk) in response to the querying process in the virtualmachine 212. These configuration characteristics of the virtual disk maybe used in techniques described below to “reclaim” storage space fromthe virtual disk back to the LUN in which the virtual disk is stored asa file by issuing an “UNMAP” command to the virtual disk. As depicted inFIG. 4, at step 402, guest OS 216 issues a SCSI Read Capacity command(e.g., 16 bit version of the command) to the virtual disk. The responseof the virtual disk, received at step 404, includes an indication ofwhether or not the virtual disk supports thin provisioning as indicatedby the setting of a bit (which, in one embodiment, is known as the “thinprovisioning enabled (TPE) bit). If, at step 406, guest OS 216determines that the virtual disk supports thin provisioning (e.g., theTPE bit is set), the method continues on to step 408. If, at step 406guest OS 216 determines that the virtual disk does not support thinprovisioning (e.g., the TPE bit is not set), the method ends.

At step 408, guest OS 216 records the virtual disk's support for thinprovisioning and issues a SCSI Inquiry command (e.g., utilizing the 0xB0“Vital Product Data” code as the type of inquiry in one embodiment) tothe virtual disk. The response of the virtual disk, received at step 410and recorded at step 412, includes an indication as to whether or notvirtual disk supports an “UNMAP” command (in some embodiments, indicatedby a UNMAP “bit”) and, if there is support, the response also includes areport of several parameters to be used with UNMAP commands. In itssimplest form, in one embodiment, an UNMAP command specifies a list ofblocks that are to be unmapped by the virtual disk and released to theLUN in which the virtual disk is stored. In one such embodiment, theparameters reported include V_(G), a granularity at which hypervisor 208manages data, V_(offset), an alignment parameter expressed as an offsetat which hypervisor 208 prefers to receive UNMAP commands, and N_(MAX)_(—) _(V), a maximum number of <offset, length> pairs that can bespecified with a single UNMAP command. V_(G), and V_(offset), for thevirtual disk are analogous to D_(G), and D_(offset) for the LUN and arethus used similarly to D_(G), and D_(offset) as previously discussed.

FIG. 5 is a flow diagram that illustrates a method of storage spacereclamation that is initiated by a VM, e.g., when a VM deletes a filestored in its virtual disk. It should be recognized that a file isrepresented in the virtual disk (e.g., which itself is a file stored ina LUN) as a series of file blocks which may or may not be contiguous,and that the method of FIG. 5 is performed for each set of contiguousfile blocks, referred to herein as a “file segment.” In one embodiment,the steps of FIG. 5 are performed by a user-level process developed tomonitor and identify free file system blocks relating to deleted filesin guest OS 216. For example, such a monitoring process may perform themethod of FIG. 5 at periodic intervals upon recognizing free blocksrelating to recently deleted files or may be notified by guest OS 216upon a deletion of a file from the virtual disk. At step 508, the UNMAPbit received in step 410 of FIG. 4 as published by the virtual disk isexamined. If this bit is not set, the method ends after suchdetermination. If, at step 508, the UNMAP bit published by the virtualdisk is set, the method continues onto step 510 where the length of afile segment of the deleted file (or alternatively, the length of anumber contiguous free file system blocks as indicated by guest OS 216)starting at an offset that complies with (e.g., is a multiple of) theoffset published by the virtual disk, V_(offset) (the length of such afile segment hereafter being referred to as “L1”), is determined.Therefore, a file segment that is not naturally aligned with V_(offset)is made to align with V_(offset) by carrying out this step. After L1 isdetermined at step 510, L1 is compared with the granularity published bythe virtual disk, V_(G), at step 514. If L1<V_(G), then, at step 516, itis determined that the file segment does not include enough contiguousfile blocks to support an UNMAP command to the virtual disk and as such,the file blocks of the file segment are remembered (e.g., identifyingthe file blocks in a special data structure) for possible coalescingwith other free blocks that are contiguous with such file blocks thatmay be subsequently identified by the monitoring process. Step 516 iscarried out because the file blocks that are remembered may becontiguous with file blocks from other file segments whose L1 is alsoless than V_(G). If so, the file blocks are coalesced for possibleinclusion in a single UNMAP command that adheres to the granularitypublished by the virtual disk. However, the file blocks that areremembered are also monitored for writes (e.g., by the monitoringprocess), and are no longer remembered (e.g., removed from the specialdata structure) if a write is issued thereto (since such file blockswould no longer be free for an UNMAP command). As indicated by thedashed arrow to step 510, the length, L1, of coalesced file blocks aredetermined at step 510. At step 514, L1 is checked once more to see ifthey meet the condition, L1<V_(G).

If L1 is greater than or equal to V_(G), the <offset, length> descriptorfor use with the UNMAP command is generated at step 518. Then, at step520, it is determined whether there are more file segments to process.If there are, the flow returns to step 510. If there are no more, theUNMAP command with a string of one or more <offset, length> descriptorsis generated and sent to the virtual disk at step 522. If the number ofdescriptors generated at step 518 is greater than the maximum numberpublished by the virtual disk, N_(MAX) _(—) _(V), the UNMAP command issplit into multiple UNMAP commands and sent separately to the virtualdisk. The method ends after step 522.

For example, if V_(G)=1 MB and V_(offset)=4 KB, and a file segmentanalyzed at step 510 began at an address corresponding to 5 KB from thebeginning of the virtual disk and had a length, L1, of 1.5 MB, then thecorresponding descriptor for this file segment generated for the UNMAPcommand would be <8 KB, 1 MB> so that the descriptor complies with thegranularity and alignment parameters published by the virtual disk. Thatis, the virtual disk is unable to unmap the beginning 3 KB portion ofthe file segment from 5 KB to 8 KB because that portion of the filesegment does not begin at an address that is a multiple of V_(offset)(i.e., 4 KB). Similarly, the virtual disk is unable to map the tailportion of the file segment (i.e., approximately the last 0.5 MB)because the tail portion falls within a second 1 MB portion of the filesegment and the virtual disk can only unmap in multiples of 1 MB.

FIG. 6 is a flow diagram that illustrates a method of storage spacereclamation that is initiated by hypervisor 208 in response to an UNMAPcommand from a VM (e.g., as received from step 522 of FIG. 5). At step602, hypervisor 208, in particular SCSI virtualization layer 228,receives the UNMAP command from the VM on behalf of the virtual disk andtranslates the UNMAP command into a VMFS file offset and length, whichconceptually correspond to a series of VMFS blocks, which may or may notbe contiguous. The VMFS blocks are hereafter referred to as free VMFSblocks and each contiguous segment of free VMFS blocks is referred to asa VMFS block segment. Then, a determination is made at step 604 whetherthe free VMFS blocks should be kept allocated. For example, when avirtual disk is not thinly provisioned and instead is pre-allocated, itwould be determined at step 604 to keep the free VMFS blocks allocated.Thus, if the free VMFS blocks should be kept allocated, the metadata(i.e., inode) of the VMFS file that represents the virtual disk isupdated so that the free VMFS blocks are indicated as “to be zeroed” asdescribed in U.S. patent application Ser. No. 12/050,805, entitled“Efficient Zeroing of File Data Blocks” (Attorney Docket No. A123),filed Mar. 18, 2008, the entire contents of which are incorporated byreference herein. The method ends after step 605. If, on the other hand,it is determined at step 604 that the free VMFS blocks should bedeallocated, the free VMFS blocks are deallocated at step 606 from theVMFS file that represents the virtual disk. This deallocation reducesthe physical size of the blocks allocated to the virtual disk (asrecorded in the inode of the VMFS file corresponding to the virtualdisk) even though the logical size remains the same. As part of thisdeallocation, the bitmap data structure managed by VMFS 230 is updatedto indicate the deallocated VMFS blocks are now free. It should be notedthat because this deallocation essentially releases free blocks from thevirtual disk (e.g., a thinly provisioned virtual disk) back to the LUN,it represents a satisfaction of the UNMAP command from the VM on thevirtual disk, as performed by hypervisor 208. However, in certainembodiments, it may be further desired to determine, at this juncture,whether the LUN itself, which has just received free blocks back fromthe virtual disk and which is a thinly provisioned LUN, may be able torelease such free blocks back to its underlying storage array (e.g., sothat such free blocks can be utilized by another LUN).

In some embodiments, hypervisor 208 may desire to reuse the free VMFSblocks (e.g., allocate such free blocks to another virtual disk). Thischeck is made at step 608. If it is determined that hypervisor 208desires to reuse the free VMFS blocks, the method ends. If, on the otherhand, it is determined at step 608 that hypervisor 208 does not desireto reuse the free VMFS blocks at the current time, the UNMAP bitpublished by the LUN that stores the free VMFS blocks is examined atstep 610 to determine whether the LUN may be able to release the freeVMFS blocks back to its underlying storage array (e.g., so that suchfree blocks can be utilized by another LUN). If this bit is not set, themethod ends after such determination. If, at step 610, the UNMAP bitpublished by the LUN is set, the method continues onto step 612 wherethe length of one VMFS block segment starting at an offset that complieswith the offset published by the LUN, D_(offset) (the length hereafterbeing referred to as “L2”), is determined. Therefore, a VMFS blocksegment that is not naturally aligned with D_(offset) is made to alignwith D_(offset) by carrying out this step. After L2 is determined atstep 612, L2 is compared with the granularity published by the LUN,D_(G). If L2<D_(G), then the VMFS blocks in the VMFS block segment areremembered (e.g., identifying the VMFS blocks in a special datastructure) for possible coalescing and writes thereto monitored at step616. Step 616 is carried out because the VMFS blocks that are rememberedmay be contiguous with VMFS blocks from other VMFS block segments whoseL2 is less than D_(G). If so, the VMFS blocks are coalesced for possibleinclusion in a single UNMAP command that adheres to the granularitypublished by the LUN. However, the VMFS blocks that are remembered aremonitored for writes, and are no longer remembered (e.g., removed fromthe special data structure) if a write is issued thereto. As indicatedby the dashed arrow to decision block 612, coalesced VMFS blocks arechecked to see if they meet the condition, L2<D_(G).

If L2 is greater than or equal to D_(G), the <offset, length> descriptorfor use with the UNMAP command is generated at step 618. Then, at step620, it is determined whether there are more VMFS block segments toprocess. If there are, the flow returns to step 612. If there are nomore, the UNMAP command with a string of one or more <offset, length>descriptors is generated and sent to the LUN at step 622. If the numberof descriptors generated at step 618 is greater than the maximum numberpublished by the virtual disk, N_(MAX) _(—) _(D), the UNMAP command issplit into multiple UNMAP commands and sent separately to the LUN. Themethod ends after step 622.

For example, if one VMFS block segment is described by <8 KB, 1 MB>, andif D_(G)=32 KB and D_(offset)=16 KB, the UNMAP command is issued withthe descriptor <16 KB, (1 MB=KB)> so that the descriptor complies withthe granularity and alignment parameters published by the LUN. That is,the LUN is unable to unmap the beginning 8 KB portion of the VMFS blocksegment from 8 KB to 16 KB because that portion of the file segment doesnot begin at an address that is a multiple of D_(offset) (i.e., 16 KB).Similarly, the LUN is unable to map the tail portion of the VMFS blocksegment (i.e., approximately the last 24 KB) because the tail portion istoo small to comply with the granularity of 32 KB.

FIG. 7 is a flow diagram that illustrates a method of storage spacereclamation that is initiated by hypervisor 208 in response to an UNLINKcommand from management server 148. The UNLINK command is issued toinform hypervisor 208 to delete a file or set of files that aremaintained by VMFS 230 and to issue an UNMAP command to deallocate theVMFS blocks corresponding to these files. For example, after StoragevMotion™ is carried out, as a result of which a set of files (e.g.,comprising or including a virtual disk, for example) associated with aVM has been migrated from a source LUN to a destination LUN, the UNLINKcommand may be issued by management server 148 to delete the files anddeallocate the VMFS blocks corresponding to these files, from the sourceLUN. This method begins at step 702, where hypervisor 208 identifies theVMFS block segments corresponding to the deleted file or deleted set offiles by examining the inodes of the file or files. The remaining stepsillustrated in FIG. 7 are identical to the same numbered steps of FIG. 6and reference is made to the description given above for these steps.

FIG. 8 is a flow diagram that illustrates a method of performingcompatibility checks prior to initiating a process to live migrate a VMfrom a source host to a destination host. These compatibility checks arecarried out by management server 148 because a VM that is supported by athinly provisioned virtual disk should preferably not be migrated to ahypervisor on the destination host that is not capable of providingthinly provisioned virtual disks with UNMAP support. These checks may beperformed in various ways including checking the version number of thehypervisor. For example, if the version number of the hypervisor on thedestination host is greater than or equal to the lowest version numberof the hypervisor that supports thin provisioning, live migration fromthe source host to the destination host is permitted. Live migration isdisallowed otherwise.

Referring to FIG. 8, at step 802, the VM to be live migrated isidentified. It is assumed for purposes of illustration that this VM issupported by a thinly provisioned virtual disk. At step 808, a check ismade to see if the hypervisor on the destination host also supportsthinly provisioned virtual disks. In one embodiment, this check is madebased on version numbers as described above. If the check fails, themethod loops through steps 810 and 812 until a destination host with acompliant hypervisor is found. If there are no compliant hypervisors,the method terminates. If a compliant hypervisor is found, livemigration of the VM is initiated by management server 148 at step 820.

FIG. 9 is a flow diagram that illustrates a method of notifyingmanagement server 148 that a LUN has reached a certain threshold in usedcapacity. This notification is given so that management server 148 canemploy remedial measures such as provisioning more space to the LUN,deleting unused files stored in the LUN using the UNLINK command, ormigrating workloads (e.g., virtual disks of VMs) from the LUN to anotherLUN, followed by invoking the UNLINK command on the migrated workloads.Steps 902 and 904 are carried out by the storage array. Step 906 iscarried out by the hypervisor.

Step 902 shows that the storage array is continually monitoring whethera LUN's storage capacity has reached or passed a certain thresholdlevel. If it determines that this condition is met, it issues a softerror message to the hypervisor at step 904 with the LUN ID. Forexample, any write operation to the LUN that results in the LUNexceeding its threshold level causes the storage array to issue the softerror message to the hypervisor. At step 906, the hypervisor, uponreceiving this soft error message, issues a soft error message tomanagement server 148. The soft error message includes the LUN ID andthe VMFS ID so that management server 148 can employ remedial measuresnoted above.

FIG. 10 is a flow diagram that illustrates a method of performingcorrective measures by management server 148 upon receiving the softerror message generated in step 906 of FIG. 9. At step 1002, managementserver 148 receives the soft error message. Then, at step 1004,management server 148 examines configuration settings of VMs that aresupported by the LUN that is nearing capacity. According to embodimentsof the present invention, one of the configuration settings that can bespecified upon deployment of a VM is what remedial measures should betaken when the LUN supporting the VM is nearing capacity. Three of thechoices are Storage vMotion™, snapshot, and power off. After thedecision block at 1006, step 1008 is executed if Storage vMotion™ isspecified in such a setting; step 1010 is executed if snapshot isspecified in such a setting; and step 1012 is executed if power off isspecified in such a setting. At step 1008, management server 148initiates Storage vMotion™ between the source LUN (i.e., the LUN nearingcapacity) and the destination LUN. After completion of Storage vMotion™,the UNLINK command is used to delete the migrated files from the sourceLUN. At step 1010, a snapshot of the VM files is created on adestination LUN and the VM files on the source LUN is marked read-only.At step 1012, management server 148 powers off the VM.

If no remedial measures are employed or they are deployed too slowly, orin spite of the remedial measures being deployed, the LUN may run out ofspace when executing a write operation. Under this condition, a harderror message is issued by the storage array. This error messageincludes an ID of the write operation that caused the error condition sothat the VM that issued the write operation can be taken down until morespace is provisioned to the LUN or additional space in the LUN isreclaimed. By taking down only the VM that caused the error, VMisolation is preserved and the other VMs that are employing the same LUNfor storage can remain operational.

FIG. 11 is a flow diagram that illustrates a method of detecting andhandling an error caused when a LUN runs out of space while performing awrite operation. Steps 1102, 1104, and 1106 are carried out by thestorage array. Step 1108 is carried out by the hypervisor. At step 1102,the storage array receives and executes a write operation on the LUN. Ifduring execution of the write operation, the LUN runs out of space asdetermined at decision block 1104, a hard error message is issued to thehypervisor at step 1106. The error message includes an ID of the writeoperation that caused the error. At step 1108, the hypervisordeactivates the VM that issued the write operation. As under normalcircumstances, the deactivation of the VM will result in an alertmessage being transmitted to management server 148, and managementserver 148 can implement remedial measures in response thereto beforereactivating the VM that has been deactivated. The remedial measuresinclude provisioning more space to the LUN, and reclaiming additionalspace in the LUN by way of migrating the virtual disk of the VM that hasbeen deactivated (or even virtual disks of other VMs) to another LUN,followed by invoking the UNLINK command on the migrated virtual disks,powering off certain VMs, or deleting other files.

FIG. 12 is a flow diagram that illustrates a method of retroactivelyreclaiming storage space from a LUN that operates as a logical blockdevice for a hypervisor that has been upgraded from a version that doesnot have the system software interface according to one or moreembodiments of the present invention to a version that does. This methodis carried out after the hypervisor upgrade has been carried out. Atstep 1202, the hypervisor determines the total size of its free VMFSblocks in the LUN. At step 1204, the hypervisor creates a temporary filethat is X % of the total size of its free VMFS blocks in the LUN. Atstep 1206, the temporary file is deleted and the hypervisor issues anUNMAP command for all blocks of the temporary file. As a result, thebitmap that the hypervisor maintains to track usage of all VMFS blocksis updated so that the VMFS blocks corresponding to the temporary fileare indicated as free in the bitmap. The X % parameter is configurableand if equal to 100%, all of the hypervisor's free VMFS blocks will bedeallocated from the LUN, reducing the physical size of the LUN to itstheoretical minimum. However, during the time the method of FIG. 12 iscarried out, there may be writes to the LUN that require additionalspace. Therefore, a certain amount of space, which can be determinedempirically, is kept free to handle such writes.

In one or more embodiments of the present invention, the commands issuedby the hypervisor to the LUN and by the guest operating system to thevirtual disk, including SCSI Read Capacity and SCSI Inquiry, and errorsissued by the storage array to the hypervisor, such as the soft errordescribed in conjunction with FIG. 9 and the hard error described inconjunction with FIG. 11, are part of the set of commands and errorcodes in the T10 SCSI protocol.

Although one or more embodiments have been described herein in somedetail for clarity of understanding, it should be recognized thatcertain changes and modifications may be made without departing from thespirit of the invention.

The various embodiments described herein may employ variouscomputer-implemented operations involving data stored in computersystems. For example, these operations may require physical manipulationof physical quantities—usually, though not necessarily, these quantitiesmay take the form of electrical or magnetic signals, where they orrepresentations of them are capable of being stored, transferred,combined, compared, or otherwise manipulated. Further, suchmanipulations are often referred to in terms, such as producing,identifying, determining, or comparing. Any operations described hereinthat form part of one or more embodiments of the invention may be usefulmachine operations. In addition, one or more embodiments of theinvention also relate to a device or an apparatus for performing theseoperations. The apparatus may be specially constructed for specificrequired purposes, or it may be a general purpose computer selectivelyactivated or configured by a computer program stored in the computer. Inparticular, various general purpose machines may be used with computerprograms written in accordance with the teachings herein, or it may bemore convenient to construct a more specialized apparatus to perform therequired operations.

The various embodiments described herein may be practiced with othercomputer system configurations including hand-held devices,microprocessor systems, microprocessor-based or programmable consumerelectronics, minicomputers, mainframe computers, and the like.

One or more embodiments of the present invention may be implemented asone or more computer programs or as one or more computer program modulesembodied in one or more computer readable media. The term computerreadable medium refers to any data storage device that can store datawhich can thereafter be input to a computer system--computer readablemedia may be based on any existing or subsequently developed technologyfor embodying computer programs in a manner that enables them to be readby a computer. Examples of a computer readable medium include a harddrive, network attached storage (NAS), read-only memory, random-accessmemory (e.g., a flash memory device), a CD (Compact Discs)—CD-ROM, aCD-R, or a CD-RW, a DVD (Digital Versatile Disc), a magnetic tape, andother optical and non-optical data storage devices. The computerreadable medium can also be distributed over a network coupled computersystem so that the computer readable code is stored and executed in adistributed fashion.

Although one or more embodiments of the present invention have beendescribed in some detail for clarity of understanding, it will beapparent that certain changes and modifications may be made within thescope of the claims. Accordingly, the described embodiments are to beconsidered as illustrative and not restrictive, and the scope of theclaims is not to be limited to details given herein, but may be modifiedwithin the scope and equivalents of the claims. In the claims, elementsand/or steps do not imply any particular order of operation, unlessexplicitly stated in the claims.

Virtualization systems in accordance with the various embodiments, maybe implemented as hosted embodiments, non-hosted embodiments or asembodiments that tend to blur distinctions between the two, are allenvisioned. Furthermore, various virtualization operations may be whollyor partially implemented in hardware. For example, a hardwareimplementation may employ a look-up table for modification of storageaccess requests to secure non-disk data.

Many variations, modifications, additions, and improvements arepossible, regardless the degree of virtualization. The virtualizationsoftware can therefore include components of a host, console, or guestoperating system that performs virtualization functions. Pluralinstances may be provided for components, operations or structuresdescribed herein as a single instance. Finally, boundaries betweenvarious components, operations and data stores are somewhat arbitrary,and particular operations are illustrated in the context of specificillustrative configurations. Other allocations of functionality areenvisioned and may fall within the scope of the invention(s). Ingeneral, structures and functionality presented as separate componentsin exemplary configurations may be implemented as a combined structureor component. Similarly, structures and functionality presented as asingle component may be implemented as separate components. These andother variations, modifications, additions, and improvements may fallwithin the scope of the appended claims(s).

We claim:
 1. A method of determining features of a logical block devicefor a computer system, comprising: querying the logical block devicewith a first query and receiving a response thereto from the logicalblock device that indicates whether or not the logical block device isthinly provisioned; and querying the logical block device with a secondquery and receiving a response thereto from the logical block devicethat indicates whether or not the logical block device supports an unmapcommand, wherein the unmap command is a command to deallocate blockspreviously allocated to the logical block device.
 2. The method of claim1, wherein the response to the second query further includes analignment value and a granularity value, the alignment value specifyingan offset at which the logical block device performs space reclamationoperations, and the granularity value specifying a unit size at whichthe logical block device performs space reclamation operations.
 3. Themethod of claim 2, wherein the response to the second query furtherincludes an integer value that represents the maximum number ofarguments that can be included in a single unmap command.
 4. The methodof claim 2, wherein the logical block device is a logical unit number(LUN) and the computer system includes a file system that is interfacedwith the logical block device.
 5. The method of claim 4, wherein thecomputer system has a virtual machine executing therein and the filesystem is a component of a virtualization software that supportsexecution of the virtual machine.
 6. The method of claim 5, wherein thevirtualization software layer emulates physical hardware for the virtualmachine including a virtual disk that is managed as a file stored in thelogical block device.
 7. The method of claim 6, further comprising:querying, from within the virtual machine, the virtual disk with a thirdquery and receiving a response thereto from the virtual disk thatindicates whether or not the virtual disk is thinly provisioned; andquerying, from within the virtual machine, the virtual disk with afourth query and receiving a response thereto from the virtual disk thatindicates whether or not the virtual disk supports an unmap command,wherein the unmap command is a command to deallocate blocks previouslyallocated to the virtual disk.
 8. The method of claim 7, wherein theresponse to the fourth query further includes an alignment value and agranularity value, the alignment value specifying an offset at which thelogical block device performs space reclamation operations, and thegranularity value specifying a unit size at which the logical blockdevice performs space reclamation operations.
 9. The method of claim 8,wherein the response to the fourth query further includes an integervalue that represents the maximum number of arguments that can beincluded in a single unmap command issued to the virtual disk.
 10. Themethod of claim 1, wherein the computer system is a virtual machineexecuting in a host and the logical block device is a virtual disk forthe virtual machine, and the virtual machine includes a file system thatis interfaced with the virtual disk.
 11. The method of claim 10, whereinthe host includes a virtualization software that supports execution ofthe virtual machine and emulates physical hardware for the virtualmachine including the virtual disk.
 12. The method of claim 11, whereinthe response to the second query further includes an alignment value anda granularity value, the alignment value specifying an offset at whichthe logical block device performs space reclamation operations, and thegranularity value specifying a unit size at which the logical blockdevice performs space reclamation operations.
 13. The method of claim12, wherein the response to the second query further includes an integervalue that represents the maximum number of arguments that can beincluded in a single unmap command.
 14. A non-transitory computerreadable storage medium comprising instructions executable by a computersystem to carry out a method of determining features of a logical blockdevice for the computer system, said method comprising: querying thelogical block device with a first query and receiving a response theretofrom the logical block device that indicates whether or not the logicalblock device is thinly provisioned; and querying the logical blockdevice with a second query and receiving a response thereto from thelogical block device that indicates whether or not the logical blockdevice supports an unmap command, wherein the unmap command is a commandto deallocate blocks previously allocated to the logical block device.15. The non-transitory computer readable storage medium of claim 14,wherein the response to the second query further includes an alignmentvalue and a granularity value, the alignment value specifying an offsetat which the logical block device performs space reclamation operations,and the granularity value specifying a unit size at which the logicalblock device performs space reclamation operations.
 16. Thenon-transitory computer readable storage medium of claim 15, wherein theresponse to the second query further includes an integer value thatrepresents the maximum number of arguments that can be included in asingle unmap command.
 17. The non-transitory computer readable storagemedium of claim 15, wherein the logical block device is a logical unitnumber (LUN) and the computer system has a file system that isinterfaced with the logical block device.
 18. The non-transitorycomputer readable storage medium of claim 15, wherein the computersystem is a virtual machine to be executed in a host and the logicalblock device is a virtual disk for the virtual machine.
 19. A computersystem interfaced with a logical block device, comprising: an interfacecomponent that interfaces the computer system to the logical blockdevice; and a processor and memory, wherein the processor is programmedto query the logical block device with a first query and receive aresponse thereto from the logical block device that indicates whether ornot the logical block device is thinly provisioned, and query thelogical block device with a second query and receive a response theretofrom the logical block device that indicates whether or not the logicalblock device supports an unmap command, wherein the unmap command is acommand to deallocate blocks previously allocated to the logical blockdevice.
 20. The computer system of claim 19, wherein the response to thesecond query further includes an alignment value and a granularityvalue, the alignment value specifying an offset at which the logicalblock device performs space reclamation operations, and the granularityvalue specifying a unit size at which the logical block device performsspace reclamation operations.
 21. The computer system of claim 20,wherein the response to the second query further includes an integervalue that represents the maximum number of arguments that can beincluded in a single unmap command.
 22. The computer system of claim 20,wherein the processor is programmed to execute a virtual machine and avirtual disk for the virtual machine, and wherein the virtual machinequeries the virtual machine with a third query and receives a responsethereto from the virtual disk that indicates whether or not the virtualdisk is thinly provisioned, and queries the virtual disk with a fourthquery and receive a response thereto from the virtual disk thatindicates whether or not the virtual disk supports an unmap command,wherein the unmap command is a command to deallocate blocks previouslyallocated to the virtual disk.
 23. The computer system of claim 20,wherein the response to the fourth query further includes an alignmentvalue and a granularity value, the alignment value specifying an offsetat which the logical block device performs space reclamation operations,and the granularity value specifying a unit size at which the logicalblock device performs space reclamation operations.
 24. The computersystem of claim 23, wherein the response to the fourth query furtherincludes an integer value that represents the maximum number ofarguments that can be included in a single unmap command issued to thevirtual disk.
 25. A computer system comprising: a plurality of hosts,each having virtualization software to support execution of one or morevirtual machines, the hosts including first and second hosts each havingvirtualization software of a first type that supports thinly provisionedvirtual disks and an unmap command, wherein the unmap command is acommand for deallocating blocks previously allocated to a thinlyprovisioned virtual disk, and a third host having virtualizationsoftware of a second type that does not support thinly provisionedvirtual disks or the unmap command; and a management server for thevirtual machines having a resource scheduling software executed thereinto recommend live migration of virtual machines between two hosts otherthan from a host having a virtualization software of the first type to ahost having a virtualization software of the second type.
 26. Thecomputer system of claim 25, wherein each of the hosts having avirtualization software of the first type includes: an interfacecomponent that interfaces the host to a logical block device; and aprocessor and memory, wherein the processor is programmed to query thelogical block device with a first query and receive a response theretofrom the logical block device that indicates whether or not the logicalblock device is thinly provisioned, and query the logical block devicewith a second query and receive a response thereto from the logicalblock device that indicates whether or not the logical block devicesupports an unmap command, wherein the unmap command is a command todeallocate blocks previously allocated to the logical block device. 27.The computer system of claim 26, wherein the response to the secondquery further includes an alignment value and a granularity value, thealignment value specifying an offset at which the logical block deviceperforms space reclamation operations, and the granularity valuespecifying a unit size at which the logical block device performs spacereclamation operations.
 28. The computer system of claim 27, wherein theresponse to the second query further includes an integer value thatrepresents the maximum number of arguments that can be included in asingle unmap command.